Humans manoeuvre easily over un-even terrain. To maintain smooth and efficient gait the motor system needs to adapt the locomotor output to the walking environment. In the present study we investigate the role of sensory feedback in adjusting the soleus muscle activity during over-ground walking in 19 healthy volunteers. Subjects walked unrestrained over a hydraulically actuated platform. On random trials the platform was accelerated downwards at 0.8g unloading the plantar flexor muscles in mid or late stance. The drop of the platform resulted in a significant depression of the soleus muscle activity of -17.9%(SD2) and -21.4%(SD2), with an onset latency of 49ms(SD1) and 45ms(SD1) in mid and late stance, respectively. Input to the vestibular apparatus, i.e. the head acceleration, occurred at a latency 10.0ms(SD2.4) following the drop and ankle dorsiflexion velocity was decreased starting 22ms(SD15) after the drop. To investigate the role of length and velocity sensitive afferents on the depression in soleus muscle activity, the ankle rotation was arrested by using an ankle foot orthotic as the platform was dropped. Preventing the ankle movement did not significantly change the soleus depression in late stance ( 18.2%(SD15) ), while the depression in mid stance was removed ( +4.9%(SD13) ). It is concluded that force feedback from ankle extensors increases the locomotor output through positive feedback in late stance. In mid-stance the effect of force feedback was not observed, suggesting that spindle afferents may have a more significant effect on the output during this phase of the step cycle.